I have yet to put my SLR in an enclosure and dive with it. But I do own the Samyang 14mm lens and shoot it on land with my 5DII. I can tell you it's a spectacular lens. I also have the 17-40 and the 14mm blows it away in corner sharpness and lacking any CA whatsoever.

I'm sure you could pick a hyperfocal and dive - it's impossible to focus through the viewfinder as everything always looks in focus. I use the 10x live view. Pick an aperture (f8+) and you'll have a pretty deep hyperfocal and treat it as focus free.